The invention relates to a method of controlling an image capturing system comprising an interface for receiving an external trigger to capture an image, and an image capturing device provided with a photosensitive area and an array of pixel cells, each pixel cell including a device for generating a signal indicative of the intensity of light falling on an associated part of the photosensitive area, which image capturing device is further provided with readout circuitry for generating an array of pixel values to capture an image frame at a set spatial resolution, such that each pixel value represents an integral of the signal or signals generated in at least one of the pixel cells in an associated one of a number of areas over an exposure time interval, the number of areas being determined by the set spatial resolution, the areas together covering a region of the photosensitive area corresponding to a region in the image.
The invention also relates to an image capturing system.
The invention also relates to a method of forming a combined final image from a plurality of image frames, including the steps of: obtaining a first and at least one further array of intensity values, each array of intensity values encoding light intensity levels at each of a respective number of pixel positions in the respective image frame, the number determining the spatial resolution of the image frame concerned, generating a set of derived arrays of intensity values, each derived array being based on a respective one of the obtained arrays of intensity levels and encoding light intensity levels at each of a common number of pixel positions in at least a region of overlap of the respective image frames, generating an array of combined intensity values, each element in the array based on a sum of intensity values represented by the corresponding element in each of the respective derived arrays of intensity values, and providing an array of intensity values encoding the combined final image, the array being based on the array of combined intensity values.
The invention also relates to an image processing system.
The invention also relates to a digital camera.
The invention also relates to a computer program.
The aforementioned application describes a digital camera. The camera can be used in a substantially stationary position to capture a sequence of images and to derive a sequence of corresponding frames of pixel values representing the images. Each image is underexposed on purpose. The images are adjusted prior to forming them into a combined final image. The combined final image is formed by summing the values of corresponding pixels in the adjusted images. The combined final image may therefore be formed from underexposed images, but is itself sufficiently bright, as well as having good spatial resolution. The adjustment is used to prevent the combined final image from being blurred.
A problem associated with capturing a series of underexposed image frames for later combination is due to the types of image capturing devices available for use. Generally, these either have pixel cells comprising Charge Coupled Devices (CCDs) or are made with Complementary Metal Oxide Semiconductor (CMOS) sensors, in both cases with associated read-out circuitry. In particular when CCD arrays are used, the readout time, i.e. the time needed by the read-out circuitry to generate the array of pixel values encoding a frame, is very long. The time needed to capture a series of consecutive image frames for subsequent formation of a combined image, is thus even longer. Setting the image spatial resolution to a lower value results in a lower spatial resolution combined image if interpolation techniques are used to increase the spatial resolution of the captured image frames. Reducing the number of captured image frames on which to base the combined final image would achieve a lower total image capture time, but at the expense of a decreased signal-to-noise ratio (SNR) of the combined final image.